the 100 is a great engine, i use a 15x6 apc on mine, will hover great.balance will be easy, i have my rx and battery right behind the fuel tank and it balances at 5.5in behind leading edge.

you will want to beef up the LG a little, i just drilled a couple holes horizontally through the LG block and ran in a couple of peices of allthread with a little epoxy in the holes and on the threads and its still all together.
(pic below)
your servo selection will be fine.
just mount the engine as far forward ao the mounting lugs on the engine are even with the end of the mount, you may have to sand(dremil) off a little of the mount where the bottom left mount screw goes.

any more questions just ask